日高山脈北部七ッ沼カールの融氷河流堆積物と恵庭a火山灰

概要

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One of the key sites of late Quaternary glaciations in the Hidaka Range is the Nanatsu-numa cirque area where glacial landforms and sediments can be correlated with the key marker tephra, En-a (17–18ka), which allow glacier extent and fluctuations around the LGM to be examined precisely. This site has been identified as a type locality for defining two stadials, Poroshiri Stadial (older) and Tottabetsu Stadial (younger), during the last glaciation in this area. We made a detailed description of the stratified sediments interpreted as glaciofluvial deposits, located immediately below the Nanatsu-numa cirque. We discovered that: (a) these deposits formed the proximal part of glacier terminus; (b) En-a is included in all layers of sediment to a depth of 15 m; and (c) several boulder layers show cyclical debris flow sedimentation facies, which were probably due to En-a covering the glacier surface causing a sudden ice melt. Consequently, all stratified sediments at this site were deposited during the younger stadial of the last glaciation in the vicinity of the glacial terminus.